Preparation of α-ketocarboxylic esters

ABSTRACT

α-Ketocarboxylic esters ##STR1## where R 1 , R 2 , R 3 , R 4  and R 5  are identical to or different from one another and are each hydrogen or straight-chain or branched alkyl, alkenyl, hydroxyl, alkoxy cycloalkyl or halogen and R 6  is unbranched lower alkyl, are prepared by reacting glycidic esters of the formula ##STR2## where R 1 , R 2 , R 3 , R 4 , R 5  and R 6  are each as defined above, in the presence of zeolites or phosphates and/or phosphoric acid or boric acid on a carrier material and/or acidic metal oxides as catalysts and are used for preparing phenylacetic esters.

The present invention relates to a process for preparingα-ketocarboxylic esters from glycidic esters in the presence of zeolitesand/or phosphates and/or phosphoric acid or boric acid on carriermaterial and/or acidic metal oxides and to the use of theseα-ketocarboxylic esters for preparing phenylacetic esters over the samecatalysts.

The glycidic ester intermediates used can be prepared in a simple mannerfrom carbonyl compounds, for example aldehydes and ketones, andα-chlorocarboxylic esters. This provides a very widely applicable methodwith which almost any desired substitution pattern can be obtained.

The synthesis of α-ketocarboxylic acids and esters was previously verycomplicated, involving for example Grignard reactions with oxalicesters.

The standard process for preparing phenylacetic esters comprises thereaction of benzyl halides with potassium cyanide to give benzylcyanides and the hydrolysis thereof to phenylacetic esters, a multistageprocess where highly toxic potassium cyanide has to be used.

As regards the subsequent use not only of the α-ketocarboxylic estersbut also of the phenylacetic esters, there are a number of options;α-ketocarboxylic acids and derivatives thereof can be used for preparingherbicidal triazinones (EP 58,885, DE-A- No. 3,106,707) or for isolatingL-amino acids (DE-A- No. 3,614,586).

It is known that epoxys can be rearranged in the presence of zeolites tocarbonyl compounds.

EP No. 100,117 describes the reaction of styrene oxide and of styreneoxides with alkyl or alkoxy substitution on the aromatic ring over atitanium-containing zeolite in the liquid phase at from 30° to 100° C.to give phenylacetaldehydes. The catalyst used for this purpose has tobe expensively prepared from costly high-purity starting materials suchas tetraalkyl orthosilicate, tetraalkyl orthotitanate andtetrapropylammonium hydroxide. High conversions are only obtained if thereaction is carried out on solvents such as methanol acetone at from 30°to 100° C. at the liquid phase with residence times of from 1 to 1.5hours. This creates increased distillation and operating expenses.Furthermore, the reaction over titanium-containing zeolites is onlypossible in the case of styrene oxide and alkylated or alkoxylatedstyrene oxides.

There is other prior art concerning the rearrangement of epoxys tocarbonyl compounds. Cyclodecanone, for example, is obtained fromepoxycyclododecane over Pd- or Rd-doped Al₂ O₃ (Neftekhimiya 16 (1976),250-254). In this paper it is expressly pointed out that zeolites areunsuitable for this reaction. Similarly, the use of A-zeolites for therearrangement of butylene oxide to butyraldehyde (55-72%) has beendescribed (Hokkaido Daigaku Kogarubu Hokoku 67 (1973), 171-178). Theselectivity leaves something to be desired. Moreover, the A-zeolitecatalyst, once deactivated by coking, is difficult to regenerate, sincethe temperatures of about 500° C. required for this purpose destroy thecrystal structure of this zeolite. Furthermore, to convert propyleneoxide into acetone or propionaldehyde over alkali metal doped X-zeolitesit is necessary to work in the absence of strongly acidic centers(Waseda Daigaku Rikogaku Kenkyusho Hokoku 67 (1974), 26-29).

It is an object of the present invention to prepare α-ketocarboxylicesters and phenylacetic esters in a simple manner from inexpensiveglycidic esters, ideally with maximum conversion, selectivity andcatalyst life.

We have found that this object is achieved by preparing α-ketocarboxylicesters of the formula (I) ##STR3## where R¹, R², R³, R⁴ and R⁵ areidentical to or different from one another and are each hydrogen orstraight-chain or branched alkyl, alkenyl, hydroxyl, alkoxy, cycloalkylor halogen and R⁶ is lower alkyl, by reacting glycidic esters of theformula II ##STR4## where R¹, R², R³, R⁴, R⁵ and R⁶ are each as definedabove, in the presence of zeolites and/or phosphates and/or phosphonicacid or boric acid on carrier material and/or acidic metal oxides ascatalysts.

The α-ketocarboxylic esters of the formula I can be used for preparingphenylacetic esters of the formula (III) ##STR5## by decarbonylation atabove 300° C. in the presence of the abovementioned catalysts.

It is also possible to convert glycidic esters of the formula (II) inone stage directly into the phenylacetic esters of the formula (III)without isolating the α-ketocarboxylic esters of the formula (I) if thereaction temperature is equal to or higher than 350° C.

Suitable R¹, R², R³, R⁴ and R⁵ of the formula (I), which may beidentical to or different from one another, are hydrogen andstraight-chain or branched alkyl of from 1 to 8 carbon atoms, inparticular of from 1 to 4 carbon atoms, such as methyl, ethyl,n/i-propyl, n/i/t-butyl or n-hexyl, and straight-chain or branchedalkenyl of from 1 to 8 carbon atoms, in particular of from 1 to 4 carbonatoms, such as ethenyl, propenyl or butenyl, and hydroxyl, and hydroxyl,and straight-chain or branched alkoxy of from 1 to 8 carbon atoms, inparticular of from 1 to 4 carbon atoms, such as methoxy, ethoxy, propoxyor butoxy, and cycloalkyl such as cyclopentyl or cyclohexyl,cyclohexenyl, halogen, such as F or Cl, trifluoromethyl,trichloromethyl, trifluoromethoxy, monofluoromethyl, fluoroethyl andfluoropropyl.

R⁶ is unbranched lower alkyl, in particular of from 1 to 4 carbon atoms,such as methyl, ethyl, n-propyl or n-butyl.

The catalysts used for the process according to the invention are acidiczeolitic catalysts. Zeolites are crystalline aluminosilicates whichpossess a highly ordered structure comprising a rigid three-dimensionalnetwork of SiO₄ and AlO₄ tetrahedra joined together by common oxygenatoms. The ratio of the Si and Al atoms: oxygen is 1:2. Theelectrovalence of the aluminum-containing tetrahedra is balanced by theinclusion of cations, for example an alkali metal or hydrogen ion, inthe crystal. Cation exchange is possible. The spaces between thetetrahedra are occupied prior to dehydration by drying or calcination bywater molecules. In zeolites, the aluminum in the lattice may also bereplaced by other elements such as B Ga, Fe, Cr, V, As, Sb, Bi or Be ormixtures thereof, or the silicon may be replaced by a tetravalentelement such as Ge, Ti, Zr or Hf.

The catalysts used for the processes according to the invention are inparticular zeolites of the mordenite group or narrow-pored zeolites ofthe erionite or chabasite type or zeolites of the faujasite type, forexample Y-, X- or L-zeolites. This group of zeolites also includes theultrastable zeolites of the faujasite type, i.e. dealuminized zeolites.Processes for preparing such zeolites are described in Catalysis byZeolites, volume 5 of Studies in Surface Science and Catalysis ed. B.Imelik et al., Elsevier Scientific Publishing Comp., 1980, page 203, andCrystal Structures of Ultra-stable Faujasites, Advances in ChemistrySeries No. 101, American Chemical Society Washington DC, pages 226 etseq. (1971), and in U.S. Pat. No. 4,512,961.

It is advantageous to use zeolites of the pentasil type. Their commonbuilding block is a five-membered ring composed of SiO₄ tetrahedra. Theyare characterized by a high SiO₂ /Al₂ O₃ ratio and by pore sizes betweenthose of the zeolites of Type A and those of Type X or Y.

These zeolites may have different chemical compositions. They arealuminosilicate, borosilicate, iron silicate, beryllium silicate,gallium silicate, chromium silicate, arsenic silicate, antimony silicateand bismuth silicate zeolites or mixtures thereof and aluminogerminate,borogerminate, gallium germinate and iron germinate zeolites or mixturesthereof. Specifically, aluminosilicate, borosilicate and iron silicatezeolites of the pentasil type are suitable for the process according tothe invention. The aluminosilicate zeolite is prepared for example froman aluminum compound, preferably Al(OH)₃ or Al₂ (SO₄)₃, and from asilicon component, preferably finely divided silicon dioxide, in anaqueous amine solution in particular in polyamines such as1,6-hexanediamine or 1,3-propane-diamine or triethylenetetraminesolution, with or in particular without the addition of alkali metal oralkaline earth metal at from 100° to 220° C. under autogenous pressure.This also includes the isotactic zeolites described in EP No. 34,727 andEP No. 46,504. The aluminosilicate zeolites obtained have an SiO₂ /Al₂O₃ ratio of from 10 to 40,000, depending on the choice of startingquantities. It is also possible to synthesize such aluminosilicatezeolites in an etherial medium, such as diethylene glycol, dimethylether, in an alcoholic medium such as methanol or 1,4-butanediol, or inwater.

The borosilicate zeolite is synthesized for example at from 90° to 200°C. under autogenous pressure by reacting a boron compound, for exampleH₃ BO₃, with a silicon compound, preferably finely divided silicondioxide, in an aqueous amine solution, in particular in1,6-hexane-diamine or 1,3-propanediamine or triethylenetetraminesolution, with or in particular without the addition of an alkali metalor alkaline earth metal. Isotactic zeolites as described in EP No.34,727 and EP No. 46,504 are also suitable. These borosilicate zeolitescan also be prepared by carrying out the reaction not in an aqueousamine solution but in an ethereal solution, for example in diethyleneglycol dimethyl ether, or in an alcoholic solution, for example1,6-hexanediol.

The iron silicate zeolite is obtained for example from an iron compound,preferably Fe₂ (SO)₄)hd 3, and a silicon compound, preferably finelydivided silicon dioxide, in an aqueous amine solution, in particular1,6-hexanediamine, with or without the addition of an alkali metal oralkaline earth metal, at from 100° to 220° C. under autogenous pressure.

The usable high-silicon zeolites (SiO₂ /Al₂ O₃ ≧10) include the ZSMtypes, ferrierite, Nu-1 and Silicalit® molecular seive, a silicapolymorph).

The aluminosilicate, borosilicate and iron silicate zeolites thusprepared, after they have been isolated, at from 100° to 160° C.,preferably at 110° C., and calcined at from 450° to 550° C., preferablyat 500° C., can be molded with a binder in a ratio of from 90:10 to40:60% by weight into extrudates or tablets. Suitable binders arevarious aluminum oxides, preferably boehmite, amorphous aluminosilicateshaving an SiO₂ /Al₂ O₃ ratio of from 25:75 to 90:5, preferably 75:25,silicon dioxide, preferably finely divided SiO₂, mixtures of finelydivided SiO₂ and finely divided Al₂ O₃, TiO₂, ZrO₂ and clay. Aftermolding, the extrudates or pellets are dried at 110° C. over 16 h andcalcined at 500° C. over 16 h.

Advantageous catalysts are also obtained on molding the isolatedaluminosilicate or borosilicate zeolite directly after drying andsubjecting it to a calcination only after molding. The synthesizedaluminosilicate and borosilicate zeolites can be used in the pure form,without binder, as extrudates or tablets, in which case the extruding orpeptizing aids used are for example ethylcellulose, stearic acid, potatostarch, formic acid, oxalic acid, acetic acid, nitric acid, ammonia,amines, silicoesters and graphite or mixtures thereof.

If the zeolite, owing to its manner of preparation, is present not inthe catalytically active, acidic H-form but for example in the Na-form,it can be completely or partially converted into the desired H-form byion exchange, for example with ammonium ions, and subsequent calcinationor by treatment with acids.

If in the course of the use of the zeolitic catalysts deactivationoccurs due to the deposition of coke, it is advisable to regenerate thezeolites by burning off the coke deposit with air or with an air/N₂mixture at from 400° to 550° C., preferably at 500° C. This restores thezeolites to their initial activity.

By partial precoking it is possible to set the activity of the catalystfor optimum selectivity in respect of the desired reaction product.

To obtain maximum selectivity, high conversion and long catalyst lives,it is advantageous to modify the zeolites. A suitable modification ofthe catalyst comprises for example doping the unmolded or moldedzeolites with metal salts by ion exchange or impregnation. The metalsused are alkali metals such as Li, Cd or K, alkaline earth metals suchas Mg, Ca or Sr, metals of main groups 3, 4 and 5 such as Al, Ga, Ge,Sn, Pb or Bi, transition metals of subgroups 4 to 8 such as Ti, Zr, V,Nb, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Sr, Ni, Pd or Pt, transitionmetals of subgroups 1 and 2 such as Cu, Ag or Zn, or rare earth metalssuch as La, Ce, Pr, Nd, Fr, Yb or U.

Advantageously, the doping is carried out by introducing the moldedzeolites initially in a riser tube and passing an aqueous or ammoniacalsolution of a halide or a nitrate of a metal as described over it atfrom 20° to 100° C. Such an ion exchange can be effected for example, onthe hydrogen, ammonium and alkali metal form of the zeolite. A furtherway of applying metal to the zeolite comprises impregnating the zeoliticmaterial with a halide, a nitrate or an oxide of the metals described inaqueous, alcoholic or ammoniacal solution. Not only ion exchange butalso impregnation are followed by at least one drying operation,alternatively by a further calcination.

A possible embodiment comprises dissolving Cu(NO₃)₂ ×3H₂ O or Ni(NO₃)₂×6H₂ O or Ce(NO₃)₃ ×6H₂ O or La(NO₃)₂ ×6H₂ O or Cs₂ CO₃ in water. Themolded or unmolded zeolite is impregnated with this solution for acertain time, say 30 minutes. The supernatant solution is stripped ofwater in a rotary evaporator. Thereafter the impregnated zeolite isdried at about 150° C. and calcined at about 550° C. This impregnationcan be carried out repeatedly in succession in order to obtain thedesired metal content.

It is also possible to prepare an aqueous Ni(CO₃)₂ solution ofammoniacal Pd(NO₃)₂ solution and to suspend the pure pulverulentzeolites therein at from 40° to 100° C. by stirring for about 24 hours.Following filtration, drying at about 150° C. and calcination at about500° C., the zeolitic material thus isolated can be further processedwith or without binders into extrudates, pellets or fluidizablematerial.

An ion exchange on the zeolites present in the H-form or ammonium formor alkali metal form can be effected by introducing the zeolitesinitially in extrudates or pellets into a column and passing an aqueousNi(NO₃)₂ solution or ammoniacal Pd(NO₃)₂ solution at a slightly elevatedtemperature of from 30° to 80° C. over it in a cycle for from 15 to 20hours. This is followed by washing with water, drying at about 150° C.and calcining at about 550° C. With some metal-doped zeolites such asthe Pd-, Cu- or Ni-doped zeolites, an aftertreatment with hydrogen isadvantageous.

A further modifying technique comprises subjecting the zeoliticmaterial, molded or unmolded, to a treatment with acids such ashydrochloric acid, hydrofluoric acid and phosphoric acid and/or steam.An advantageous procedure is to treat zeolites in powder form with 1 Nphosphoric acid at 80° C. for 1 hour, washing with water, drying at 110°C. over 16 h and calcining at 500° C. over 20 h.

In another procedure, zeolites are treated before or after molding withbinders with from 3 to 25% strength by weight, in particular from 12 to20% strength by weight, aqueous hydrochloric acid at from 60° to 80° C.,for example from 1 to 3 hours. Thereafter the zeolite thus treated iswashed with water, dried and calcined at from 400° to 500° C.

A particular form of the acid treatment comprises treating the zeoliticmaterial before it is molded with from 0.001 N to 2 N, preferably from0.05 N to 0.5 N, hydrofluoric acid for from in general 0.5 to 5,preferably from 1 to 3, hours at an elevated temperature, by refluxing.After the zeolitic material is isolated by filtration and washed, it isadvantageously dried at from 100° to 160° C. and calcined at from 450°to 600° C. In another preferred form of the acid treatment, the zeoliticmaterial, after molding with binder, is treated at elevatedtemperatures, advantageously at from 50° to 90° C., preferably at from60° to 80° C., with from 12 to 20% strength by weight hydrochloric acidfor from 0.5 to 5 hours. Thereafter the zeolitic material is washed andadvantageously dried at from 100° to 160° C. and calcined at from 450°to 600° C. An HF treatment may also be followed by an HCl treatment.

In another procedure, zeolites can be modified by applying phosphoruscompounds, such as trimethoxyphosphate, trimethoxyphosphine or primary,secondary or tertiary sodium phosphate. The treatment with primarysodium phosphate has proved particularly advantageous. Here the zeolitesare impregnated in extruded, tablet or fluidized form with aqueous NaH₂PO₄ solution, dried at 110° C. and calcined at 500° C.

Further catalysts for the process are phosphates, in particular aluminumphosphates, silicon aluminum phosphates, silicon iron aluminumphosphates, cerium phosphates, zirconium phosphates, boron phosphates,iron phosphates and mixtures thereof.

Aluminum phosphate catalysts used for the process are in particularhydrothermally synthesized aluminum phosphates which have a zeolitestructure.

Hydrothermally synthesized aluminum phosphates are for example APO-5,APO-9, APO-11, APO-12, APO-14, APO-21, APO-25, APO-31 and APO-33.Syntheses of these compounds are described in EP No. 132,708, U.S. Pat.No. 4,301,440 and U.S. Pat. No. 4,473,663.

AlPO₄ -5 (APO-5) for example is synthesized by homogeneously mixingorthophosphoric acid with pseudoboehmite (Catapol S8®) in water, addingtetrapropylammonium hydroxide, and then reacting in an autoclave atabout 150° C. under autogenous pressure for from 20 to 60 hours. TheAlPO₄ is filtered off, dried at from 100° to 160° C. and calcined atfrom 450° to 550° C.

AlPO₄ -9 (APO-9) is synthethized from orthophosphoric acid andpseudoboehmite in aqueous DABCO solution (1,4-diazabicyclo[2.2.2]octane)at about 200° C. under autogenous pressure in the course of from 200 to400 hours.

AlPO₄ -21 (APO-21) is synthesized from orthophosphoric acid andpseudoboehmite in aqueous pyrrolidone solution at from 150° to 200° C.under autogenous pressure in the course of 50 to 200 hours.

The silicon aluminum phosphates used for the process according to theinvention are for example SAPO-5, SAPO-11, SAPO-31 and SAPO-34. Thesynthesis of these compounds is described in EP No. 103,117 and U.S.Pat. No. 4,440,871. SAPOs are prepared by crystallization from aqueousmixture at from 100° to 250° C. under autogenous pressure in the courseof from 2 hours to 2 weeks, during which the reaction mixture comprisinga silicon component, an aluminum component and a phosphorus component isconverted in aqueous organoamine solutions.

SAPO-5 is obtained by mixing SiO₂ suspended in aqueoustetrapropylammonium hydroxide solution with an aqueous suspension ofpseudoboehmite and orthophosphoric acid and subsequent reaction at150°-200° C. for from 20 to 200 hours under autogenous pressure in astirred autoclave. The powder is filtered off, dried at from 110° to160° C. and calcined at from 450° to 550° C.

Phosphate catalysts used for the process also include precipitatedaluminum phosphates. Such aluminum phosphate is prepared for example bydissolving 92 g of diammonium hydrogenphosphate in 700 ml of water. 260g of Al(NO₃)₃ ×H₂ O in 700 ml of water are added dropwise over 2 hours,during which the pH is maintained at pH 8 by the simultaneous additionof 25% strength NH₃ solution. The resulting precipitate is subsequentlystirred for 12 hours, and then filtered off with suction and washed. Itis dried at 60° C. over 16 h.

Suitable boron phosphates for the process according to the invention canbe prepared for example by mixing and kneading concentrated boric acidand phosphoric acid and subsequent drying and calcination in an inertgas, air or steam atmosphere at from 250° to 650° C., preferably at from300° to 500° C.

These phosphates may also be treated with modifying components asdescribed above for zeolites by impregnation (soaking or spraying) or insome cases by ion exchange. As with the zeolite catalysts, modificationwith acids is also possible.

Suitable acidic catalysts are for example the acidic oxides of elementsof main groups III and IV and also subgroups IV, V and VI of theperiodic table, in particular oxides such as silicon dioxide in the formof silica gel, diatomaceous earth or quartz, and also titanium dioxide,zirconium dioxide, phosphorus oxides, vanadium oxides, niobium oxides,boron oxides, chromium oxides, molybdenum oxides, tungsten oxides orpumice or mixtures of these oxides. These oxides can also be doped byapplying modifying components as described above for the zeolitecatalysts. Another possible modifying technique is as with the zeolitecatalysts a treatment with acids.

It is possible to use catalysts impregnated with phosphoric acid orboric acid. Phosphoric acid or boric acid is applied for example toSiO₂, Al₂ O₃ or pumice carrier material, for example by impregnating orspraying. A catalyst which contains phosphoric acid can be obtained forexample by impregnating SiO₂ with H₃ PO₄, NaH₂ PO₄ or Na₂ HPO₄ solutionand subsequent drying or calcination. However, phosphoric acid can alsobe sprayed together with silica gel in a spray tower; this is followedby a drying step and usually by a calcination. Phosphoric acid can alsobe sprayed onto the carrier material in an impregnating mill.

The catalysts described here can be optionally used as from 2 to 4 mmextrudates or as tablets from 3 to 5 mm in diameter or as chips from 0.1to 0.5 mm in particle size or in fluidizable form.

The conversion according to the invention is preferably carried out inthe gas phase at from 100° to 500° C., preferably at from 200° to 350°C., in particular at from 250° to 300° C., under a weight hourly spacevelocity (WHSV) of from 0.1 to 20 h⁻¹, preferably of from 0.5 to 5 h⁻¹,of g of starting material per g of catalyst per hour. The reaction canbe carried out in a fixed bed or alternatively in a fluidized bed.

It is also possible to carry out the reaction in the liquid phase (bythe suspension, trickle bed or liquid phase procedure) at from 50° to200° C.

The process can be carried out under atmospheric pressure, under reducedpressure or under superatmospheric pressure, batchwise or preferablycontinuously.

Sparingly volatile or solid starting materials are used in dissolvedform, for example in THF, toluene or petroleum ether solution. Ingeneral, a dilution of starting materials with the solvents or withinert gases such as N₂, Ar or H₂ O vapor is possible.

After the reaction, the products formed are isolated from the reactionmixture in a conventional manner, for example by distillation;unconverted starting mixture is recycled, where appropriate.

EXAMPLES 1 TO 15

The reactions are carried out in the gas phase under isothermalconditions in a tubular reactor (coil, 0.6 cm internal diameter, 90 cmin length) for not less than 6 hours. The reaction products areseparated off and characterized in a conventional manner. Thequantitative determination of the reaction products and the startingmaterials is done by gas chromatography.

Catalyst A

The borosilicate zeolite of the pentasil type is prepared in ahydrothermal synthesis from 640 g of finely divided SiO₂, 122 g of H₃BO₃ and 8000 g of an aqueous 1,6-hexadiamine solution (mixture 50:50% byweight) at 170° C. under autogenous pressure in a stirred autoclave.After filtration and washing, the crystalline reaction product is driedat 100° C. over 24 h and calcined at 500° C. over 24 h. Thisborosilicate zeolite is composed of 94.2% by weight SiO₂ and 2.3% byweight of B₂ O₃.

This material is used to produce by molding with a molding aid 2 mmextrudates which are dried at 110° C. over 16 h and calcined at 500° C.over 24 h.

Catalyst B

An aluminosilicate zeolite of the pentasil type is prepared underhydrothermal conditions under autogenous pressure and at 150° C. from 65g of finely divided SiO₂ and 20.3 g of Al₂ (SO₄)₃ ×18 H₂ O in 1 kg of anaqueous 1,6-hexanediamine solution (mixture 50:50% by weight) in astirred autoclave. The crystalline reaction product is filtered off,washed, dried at 110° C. over 24 h and calcined at 500° C. over 24 h.This aluminosilicate zeolite contains 91.6% by weight of SiO₂ and 4.6%by weight of Al₂ O₃. The catalyst is molded with a molding aid into 2 mmextrudates, dried at 110° C. over 16 h and calcined at 500° C. over 24h.

Catalyst C

Catalyst C is obtained by impregnating the extrudates of Catalyst A withan aqueous Cs₂ CO₃ solution, then drying at 130° C. over 2 h andcalcining at 540° C. over 2 h. The Cs content is 0.6% by weight.

Catalyst D

Catalyst D is prepared in the same way as Catalyst C, except that Cs₂CO₃ is replaced by Ce(NO₃)₂. The Ce content is 1.8% by weight.

Catalyst E

The synthesis of AlPO₄ -5 (APO-5) is effected by stirring together 200 gof a 95% strength phosphoric acid dissolved in 325 g of H₂ O, 136 g ofboehmite and 678 g of tetrapropylammonium hydroxide (30% strength) andsubsequent reaction at 150° C. under autogenous pressure for 43 hours.The product dried at 120° C. and calcined at 500° C. over 16 h contains46.5% by weight of P₂ O₅ and 45.5% by weight of Al₂ O₃. This AlPO₄ -5 ismolded with boehmite in a weight ratio of 60:40 and 2 mm extrudates,dried at 110° C. and calcined at 500° C. over 16 h.

Catalyst F

Silicon aluminum phosphate-5 (SAPO-5) is prepared from a mixture of 200g of 98% strength phosphoric acid, 136 g of boehmite, 60 g of silica sol(30% strength), 287 g of tripropylamine and 587 g of H₂ O. This mixtureis reacted at 150° C. under autogenous pressure for 168 hours. Thecrystalline product is filtered off, dried at 120° C. and calcined at500° C. SAPO-5 contains 49.8% by weight of P₂ O₅, 33.0% by weight of Al₂O₃ and 6.2% by weight of SiO₂. SAPO-5 is molded with a molding aid into3 mm extrudates, dried at 120° C. and calcined at 500° C.

Catalyst G

Commercial zirconium phosphate Zr₃ (PO₄)₄ molded in pure form.

Catalyst H

BPO₄ is prepared by adding 49 g of H₃ BO₃ together with 117 g of H₃ PO₄(75% strength) to a kneader, evaporating off excess water and moldingthe reaction product into 3 mm extrudates. These extrudates are dried at100° C. and calcined at 350° C. Catalyst H contains 8.77% by weight of Band 28.3% by weight of P.

Catalyst I

SiO₂ commercially available as D 11-10®.

The experimental results obtained for these catalysts under whichexperimental conditions are summarized in Tables 1 and 2.

EXAMPLE 16

100 ml of methyl p-tert-butylphenylglycidate per hour are passed in thepresence of 200 l/h of N₂ at 180° C. over 1 l of catalyst A accomodatedin a tubular reactor electrically heated from the outside. The gaseousreaction products are condensed and worked up and characterized in aconventional manner. The preparation of pure methyl ester of3-[p-tert-butyl]phenylpyruvic acid is possible by conventionaldistillation. A yield of 91.8% is isolated. ##STR6##

EXAMPLE 17

100 ml of the methyl ester of 3-[p-tert-butyl]-phenylpyruvic acid arevaporized in the presence of 200 l/h of N₂ and passed at 350° C. over 1l of catalyst A (apparatus as in Example 16). Conventional working upreveals a conversion of 31% and a selectivity of 83% in respect ofmethyl p-tert-butylphenylacetate. ##STR7##

EXAMPLE 18

Example 16 is repeated, except that the temperature employed is 350° C.A mixture of methyl 3-[p-tert-butyl]-phenylpyruvate and methylp-tert-butylphenylacetate in a ratio of 65:35 is found.

                                      TABLE 1                                     __________________________________________________________________________    Conversion of methyl phenylglycidate (1) into                                 methyl 3-phenylpyruvate (2)                                                             Examples                                                                      1  2  3  4  5  6  7  8  9                                           __________________________________________________________________________    Catalysts A  B  C  D  E  F  G  H  I                                           Temperature °C.                                                                  200                                                                              200                                                                              200                                                                              200                                                                              200                                                                              200                                                                              200                                                                              200                                                                              200                                         WHSV h.sup.-1                                                                           3.0                                                                              3.0                                                                              3.0                                                                              3.0                                                                              3.0                                                                              3.0                                                                              3.0                                                                              3.0                                                                              3.0                                         Conversion of (1)                                                                       100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                         Selectivity for                                                                         92.5                                                                             88.1                                                                             93.7                                                                             89.7                                                                             78.3                                                                             86.9                                                                             81.5                                                                             84.8                                                                             80.3                                        (2)                                                                           __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Conversion of substituted methyl phenylglycidates of                          the formula (II) into substituted methyl 3-phenyl-                            pyruvates of the formula (I)                                                         Example                                                                       10   11   12    13    14   15                                          __________________________________________________________________________    Substituent                                                                          4-fluoro                                                                           4-methyl                                                                           4-methoxy                                                                           4-trifluoro                                                                         4-t-butyl                                                                          1-methyl-                                   on aromatics           methyl     4-fluoro                                    catalyst                                                                             A    A    A     A     A    A                                           temp. °C.                                                                     200  200  200   200   200  200                                         WHSV h.sup.-1                                                                        3.0  3.0  3.0   3.0   3.0  3.0                                         Conversion                                                                           100  100  100   100   100  100                                         of (II)                                                                       Selectivity                                                                          87.5 93.7 92.5  92.3  90.3 91.0                                        for (I)                                                                       __________________________________________________________________________

We claim:
 1. A process for preparing an α-ketocarboxylic ester of theformula (I) ##STR8## where R¹, R², R³, R⁴ and R⁵ are identical to ordifferent from one another and are each hydrogen or straight-chain orbranched alkyl, alkenyl, hydroxyl, alkoxy, cycloalkyl or halogen and R⁶is unbranched lower alkyl, which comprises reacting a glycidic ester ofthe formula (II) ##STR9## where R¹, R², R³, R⁴, R⁵ and R⁶ are each asdefined above, in the presence of a catalyst selected from the groupconsisting of a zeolite, a phosphate, phosphoric acid or boric acid on acarrier material, an acidic metal oxide and mixtures thereof.
 2. Theprocess of claim 1, wherein the catalyst used is an aluminosilicate,borosilicate or iron silicate zeolite of the pentasil type, a zeolite ofthe faujasite type or a mixture thereof.
 3. The process of claim 1,wherein the catalyst used is a zeolite doped with an alkali metal, atransition metal, a rare earth metal or a mixture thereof.
 4. Theprocess of claim 1, wherein the catalyst used is a phosphate of one ofthe elements B, Al, Zr, Ce, Fe or Sr or a mixture thereof.
 5. Theprocess of claim 1, wherein the catalyst used is a hydrothermallysynthesized phosphate having a zeolite structure.
 6. The process ofclaim 1, wherein the catalyst used is a hydrothermally synthesizedaluminum phosphate or silicon aluminum phosphate or silicon ironaluminum phosphate or boron aluminum phosphate.
 7. The process of claim1, wherein the catalyst used is phosphoric acid or boric acid on SiO₂,Al₂ O₃, TiO₂ or pumice as a carrier material.
 8. The process of claim 1,wherein the catalyst used is a metal oxide of one of the elements Al,Si, Ti, Zr, Be, Cr and V.